1. Field of the Invention
The present invention relates to optical pickup devices, and more particularly, to an optical-pickup device which can be adapted to both an optical disk having a thick transparent substrate and an optical pickup device having a thin transparent optical disk.
2. Description of the Background Art
As a reproducing optical disk dealing with multimedia information, a CD-ROM and the like have been put to practical use. A CD-ROM is a medium having a diameter of 12 cm, a thickness of 1.2 mm, and a track pitch of 1.6 .mu.m, on one side of which information for 540 Mbytes can be recorded. Recently, development of a digital video disk (DVD) having video information recorded on the CD-ROM using an image compression data technology such as an MPEG has been remarkable. However, the current CD-ROM can record only video data for approximately 20 minutes according to an MPEG-2 standard (data transfer rate 4 Mbps). Therefore, the current CD-ROM is insufficient in capacity to store video information for approximately two hours.
In order to solve this problem, a high density technology has been developed remarkably for making a recording density of an optical disk which is a medium for reproducing information several times as high as the current recording density. For example, an SD (super density) standard is proposed which records information for approximately 5 Gbytes on one side of a disk having the same diameter as that of the CD-ROM (12 cm). A disk according to the SD standard employs an efficient modulation system with a track pitch (D in FIG. 5) of approximately 0.725 .mu.m and the shortest pit length of approximately 0.4 .mu.m. The thickness of the SD specified disk is approximately 0.6 mm. A double-sided optical disk having two such disks affixed to each other can record information for approximately 10 Gbytes. This amount of information corresponds to video information for approximately four hours. In this SD specified disk, a program area starts at a position 24 mm apart from the center of the disk.
Further, an MMCD (multimedia compact disk) standard (which was previously called as an "HDMCD") is proposed which stores information for approximately 3.7 Gbytes on one side of a disk having the same diameter as that of the CD-ROM (12 cm). According to the MMCD standard, the track pitch is approximately 0.84 .mu.m, and the shortest pit length is approximately 0.45 .mu.m. The thickness of the MMCD specified disk is 1.2 mm. In this MMCD specified disk, a program area starts at a position 23 mm apart from the center of the disk.
In order to carry out reproduction on a disk having a high recording density, the spot diameter of a laser beam must be diaphragmed. To do so, it is considerable that the wavelength of a laser of an optical pickup device is shortened (for example, approximately 635 nm), and that the numerical aperture (NA) of an objective lens is made large (for example, approximately 0.6). However, the aberration by inclination of a disk increases in direct proportion to the cube of the numerical aperture of the objective lens. Therefore, making the numerical aperture of the objective lens too large causes decrease of an inclination margin of the disk. On the other hand, the aberration also increases in direct proportion to the thickness of the disk. Therefore, in order to solve the above described problem, an attempt is made to decrease a disk substrate in thickness. Under the condition of the same numerical aperture of an objective lens, the SD specified disk having a thickness of approximately 0.6 mm which is announced as DVD has an inclination margin two times as large as that of a conventional CD having a thickness of approximately 1.2 mm, for example. The above background art of the present invention is described in pages 88 to 100 of "Nikkei Electronics", published on Feb. 27, 1995.
An objective lens of an optical pickup device is generally designed taking the thickness of a transparent substrate of an optical disk into consideration. Therefore, when reproduction is carried out on, using an objective lens designed to be adapted to an optical disk having a certain thickness, another optical disk having a different thickness, the focal point of a laser beam is dispersed in the depth direction due to the spherical aberration, making it difficult to reproduce information favorably. For example, when reproduction is carried out on an optical disk having a thickness of approximately 1.2 mm using an objective lens designed to be adapted to an optical disk having a thickness of approximately 0.6 mm, the peak value at the center in an intensity distribution of the laser is substantially decreased on a recording surface of the optical disk, whereby the spot diameter is increased. As a result, it is difficult to carry out favorable reproduction.
In the future, coexistence of a disk having a standard density and a thickness of approximately 1.2 mm (CD, CD-ROM), a disk having a high density and a thickness of approximately 1.2 mm (MMCD), and a disk having a high density and a thickness of approximately 0.6 mm (SD) is expected. Therefore, an optical pickup device which can reproduce information on both the CD and the like and the SD, or an optical pickup device which can reproduce information on both the SD and the MMCD is desired.